Adverse neighborhood contexts predict poor mental health and externalizing behavior among adolescents, and the vast racial inequalities in neighborhood environment may play an important role in creating and maintaining racial disparities in health. However, since most research has been observational rather than experimental, it is potentially biased. Neighborhood-health research typically models point-in-time exposures of neighborhoods, in lieu of understanding neighborhood exposure trajectories, and insufficient attention has been paid to the specific contexts navigated by minority, impoverished, adolescents. Our study proposes a secondary analysis of data from a social experiment of voluntary neighborhood relocation using Section 8 Housing Vouchers in 5 cities (the Moving to Opportunity, MTO, Study), to test how and why random assignment of an offer to move to a low-poverty neighborhood unexpectedly adversely affected behavioral problems for boys over a 4-7 year period, and to inform the program's opposite gender effects on mental health. We will complement the existing data with additional neighborhood-level data collection to enrich the dataset. MTO is the only available large- scale study that has randomly assigned individuals to receive different neighborhood contexts, to support strong causal inferences of how neighborhoods shape mental health. Yet prior MTO analyses have not integrated theoretical understanding of adolescent development to motivate empirical analyses. Nor have prior analyses probed why neighborhood context may increase behavioral problems of boys, whether certain subgroups are more likely to be affected than others by neighborhood relocation, or whether there are important developmentally-sensitive periods when exposure to neighborhood deprivation may be especially influential on youth mental health. Our project proposes 3 aims: Aim 1: To test whether effects of moving to a low-poverty neighborhood on behavioral problems for boys are mediated by neighborhood characteristics, family context, or substance use. Aim 2: To examine, separately among boys and girls, whether the effect of moving to a low-poverty neighborhood on behavioral problems is modified by baseline health status, family/adolescent vulnerability, or program site. Aim 3: To compare the importance of alternative timing or sequencing patterns of exposure to neighborhood poverty on behavioral problems among boys. The project will leverage an interdisciplinary team of investigators, including fields of social epidemiology, psychiatric epidemiology, developmental psychology, demography, public policy, statistics, medicine. This project will capitalize on an experimental design, will innovatively apply causal methods to understand mediation, and will model time-varying neighborhood exposures, which will inform relevant early life course developmental timing of neighborhood exposure periods for adolescent well-being. Because of the rigorous experimental design, this project can provide strong policy-relevant evidence on the mechanisms by which social policies like Section 8 housing vouchers may influence population health and mental health.